1. Field of the Invention
The present invention generally relates to a semiconductor apparatus fabrication method, and more particularly to a semiconductor apparatus fabrication method that includes processes for forming an ultra fine pattern by using a resist pattern.
2. Description of the Related Art
Recent advances in micro-fabrication techniques have realized ultra high-speed semiconductor apparatuses in which patterns of a gate electrode and a contact hole are formed under an approximately 0.1 μm design rule. Currently, it is being considered to use a 0.05 μm design rule and further a 0.01 μm design rule. When such an ultra high-speed semiconductor apparatus is fabricated, a resist is formed on a film whereupon a pattern is formed and then the resist is exposed and developed under the desired design rule. As a result, it is possible to obtain a fine resist pattern corresponding to the design rule and form the pattern on the underlayer by using the resist pattern as a mask.
When such a fine pattern is formed, deep ultra-violet light of short wavelength is used in order to enhance the resolution for resist exposure. The deep ultra-violet light is generated by KrF excimer laser (wavelength 248 nm), ArF excimer laser (wavelength 193 nm) or the like. In a conventionally used novolak system resist, however, the deep ultra-violet light is considerably absorbed into the resist. As a result, there arises insufficient exposure at the bottom of the resist. For this reason, when an ultra-fine semiconductor apparatus is fabricated under a design rule of less than 0.1 μm, a chemically amplified resist is generally used. Since the chemically amplified resist contains a photo acid generator, it is possible to change the solubility of the chemically amplified resist to an alkaline developer. Furthermore, the chemically amplified resist has high permeability to the deep ultra-violet light.
However, when a pattern to be formed has an even narrower width of 0.05 μm or 0.01 μm, a resist pattern cannot have a stable edge in terms of not only individual patterns but also the interior of one pattern due to a light contrast problem on exposure or a resist composition nonuniformity problem. Namely, edge roughness is caused. If the edge roughness results in size variations of the resist pattern, the sizes of a micro gate electrode pattern and a contact hole also vary because the resist pattern is used as the mask to transfer a pattern on the underlayer of the resist pattern.
Conventionally, the edge roughness problem has been challenged by improving uniformity of resist materials. However, it is difficult to overcome edge roughness by simply improving resist materials with respect to current ultra-micro semiconductor apparatuses in which the pattern size is less than 0.1 μm.
There is such a way that reflow is caused by heating a resist pattern so as to alleviate the edge roughness problem. However, if the resist pattern is heated, there is a risk that the resist pattern will be deformed as a whole.
Japanese Laid-Open Patent Application No. 2001-332484 discloses a technique for radiating light of such wavelength that a resist pattern can absorb the light for a short time so as to cause local reflow on only the surface of the resist pattern. In this conventional technique, since the chemically amplified resist thereof has permeability toward the ArF excimer laser and the KrF excimer laser, it is necessary to use extremely short wavelength light. However, it is difficult to prepare an illuminant suitable to such wavelength.
Japanese Laid-Open Patent Application No. 11-145031 discloses a technique for causing local reflow on the surface of a resist pattern. In this technique, the surface of the chemically amplified resist pattern is exposed to an acidic solution or an acidic atmosphere in order to eliminate a protecting group of a resist resin on the pattern surface and decrease softening temperature on the resist pattern surface. In this conventional technique, however, if the softening temperature does not sufficiently decrease, the reflow arises not only on the pattern surface but also throughout the entire pattern. As a result, the resist pattern is deformed.
In order to make the resist pattern surface smooth, there is such a way that an ashing process is performed for the formed resist pattern surface by using oxygen plasma. In this case, however, it is impossible to avoid reduction of the pattern size. Accordingly, especially, in order to form a line and space pattern, it is necessary to increase the width of the line part thereof so as to compensate for the size reduction due to the ashing process. However, if the line part increases, the width of the space part decreases. As a result, it is necessary to perform exposure at the limit of exposure resolution, and there arise serious problems on the yield and the throughput.
As mentioned above, no effective method for improving the edge roughness of a fine resist pattern has been proposed.